The present invention relates to an electrophotographic copier having a developing device of the type using a liquid developer and, more particularly, to an electrophotographic copier capable of starting on an operation immediately even when left unused over a long period of time.
Conventional electrophotographic copiers include one which has an image carrier in the form of a photoconductive element and a developing device of the type using a liquid developer for developing a latent image electrostatically formed on the image carrier. The liquid developer consists of a solvent or carrier implemented with oil-based aliphatic hydrocarbon, and a condensed toner dispersed in the carrier and made up of a coloring agent, resin, and some additive. Examples of such a carrier are Isopar E, G, H, L, K and M (available from Esso), and Shellzole 71 and Solvesso 150 (available from Shell) which have desirable toner dispersibility, developing characteristic, and fixing characteristic. Other carriers for the above application include cyclohexane, n-hexane, n-heptane, n-nonane, n-octane, isooctane, isododecane, and ligroine. When this kind of liquid developer, i.e. , a dispersion of a toner in a carrier is used to develop a latent image electrostatically formed on the drum, some of the liquid remains on the drum after the development. Usually, the liquid so remaining on the drum is removed by a cleaning device which is located in close proximity to the drum. The cleaning device has a cleaning roller contacting the surface of the drum and supplied with the liquid from a tank, and a cleaning blade contacting the surface of the drum at the free end thereof. The cleaning roller is rotated and held in contact with the rotating drum to remove most of the toner remaining on the drum, while the cleaning blade removes the toner which the roller has failed to remove.
On completing the above-mentioned cleaning operation, the cleaning roller and cleaning blade are released from the surface of the drum in synchronism with the stop of rotation of the drum. At this instant, some liquid developer is left between the cleaning roller and cleaning blade and the drum. In general, the carriers constituted by oil-based aliphatic hydrocarbon are highly volatile. Hence, when the copier is not used over a long period of time after the cleaning operation, the carrier of the developer left on the drum volatilizes with the result that only the toner remains on and adheres to the drum due to drying. It has been customary to cope with this problem by the combination of a preheat mode and a precleaning procedure. Specifically, when the copier ended its operation is left unused after the stop of the drum and the stop of the developer supply from the tank to the developing and cleaning devices, it is caused into a preheat mode on the lapse of a predetermined period of time. Then, in the event of the next copying operation, the preheat mode is cancelled, and the liquid developer is again fed from the tank to the developing and cleaning devices to preclean the surface of the drum. During the precleaning, the cleaning roller and cleaning blade of the cleaning device are held in contact with the drum to remove the toner dried and adhered to the surface of the drum. Such precleaning usually continues about 30 seconds with the result that at least about 30 seconds is simply wasted until the copier becomes actually operable in the event of the next copying operation. This retards the time when the copier can start on an actual copying operation and thereby critically lowers the productivity of copies.
The liquid developer contains a carrier having high volatility, as stated earlier. Hence, when the copier is not used over a long period of time, the carrier volatilizes not only on the surface of the drum but also inside of the the developing and cleaning devices. Then, only the toner deposits or solidifies in the devices to form sludge, failing to have an acceptable dispersed condition in the carrier. Images developed by such a liquid developer would have white stripes, black dots and other defects while suffering from an irregular density distribution. The previously stated precleaning eliminates this problem. Specifically, by the precleaning, the liquid in the tank is stirred to maintain the toner in a good dispersed condition in the carrier. This liquid is fed from the tank to the developing and cleaning devices to set up the desirable dispersed condition of the toner in such devices also, preventing the liquid with deposited or solidified toner from being used for development. The precleaning, however, has a drawback that it wastefully consumes about 30 seconds and, therefore, prevents the copier to start on a copying operation immediately.